The present invention relates to a process for preparing rotund particles of salt-coated magnesium or magnesium alloy. The salt coating acts as a protective coating for the magnesium or magnesium alloy, i.e. protection against, for example, oxygen and moisture.
Such salt-coated metal particles as described above are suitable for use as, for example, a desulphurizing agent in the iron and steel industry, a nodularizing agent for producing ductile iron, and an alloying element with aluminum. For these purposes, the salt-coated metal particles are added to a molten metal through a lance by means of a carrier gas.
In order to ensure reliable feeding of the particles, and also prevent blockage of the lance, it is desirable that the coated metal particles have as uniform a size and shape as possible.
As is known, magnesium is an easily oxidized metal, and in finely divided form it can be pyrophoric. Also, in contact with water, magnesium can generate hydrogen. These factors result in explosion and fire hazards in the production, transport and handling of particles of magnesium or magnesium alloys.
For these reasons, it has been a normal practice, in the production of magnesium or magnesium alloy particles, for example, by centrifuging liquid metal by means of a rotating disc or perforated cup, to carry out the process in an inert atmosphere. In addition to being expensive, as a result of the requirements for an inert gas and relatively complicated apparatus, this process is not entirely satisfactory with regard to avoiding the hazard of explosion. Furthermore, in such prior art processes, there is an inability to adequately control the particle size of the particles, and also, a high amount of dust is usually generated.
U.S. Pat. No. 3,881,913 discloses a process for preparing a magnesium-containing mixture, by centrifuging molten metal during simultaneous addition of a salt mixture having a lower melting point than the melting point of the magnesium. This process is carried out in air, and the salt mixture contains alkali metal chlorides and fluorides, magnesium chloride and alkaline earth metal chlorides. The product resulting from this process is a mixture of salt-coated magnesium granules having a spherical and/or eliptical shape, and granules of the salt itself. The process has the disadvantages of insufficient control of the shape and size of the produced particles, variable thickness of the salt coating on the metal particles, and the failure to eliminate the danger of the magnesium catching fire during centrifuging.
U.S. Pat. No. 4,186,000, and U.S. Pat. No. 4,279,641, the latter being a continuation-in-part of the former, disclose a process for recovering rotund, salt-coated magnesium particles entrapped in friable matrix of sludge or slag (dross) material from magnesium electrolysis cells or holding furnaces. The process of U.S. Pat. No. 4,186,000 is based on the addition of a boron-containing dispersant to the molten matrix consisting of a mixture of electrolyte salts, magnesium metal, magnesium oxide and some impurities, stirring the mixture to achieve dispersion, followed by cooling to freeze the mixture, disintegration of the frozen mixture, and screening off the salt-coated magnesium particles. In this process, boron is used as a surface-stabilizing agent to prevent coalescence of the dispersed magnesium particles. U.S. Pat. No. 4,279,641 indicates that the use of the boron, or other dispersing aid, is optional. However, in this instance, it is necessary to keep the alkali metal chloride in the salt mixture to at least about 46%, preferably at least about 50%, and to employ salt mixtures which have a eutectic melting point at or below the melting point of magnesium, in order that the magnesium granules freeze first when the mixture is cool. Certain other requirements for the composition of the salt mixture must also be met in the event no dispersing aid is employed.
Additionally, in both U.S. Pat. No. 4,186,000 and U.S. Pat. No. 4,279,641, the stirrer used to form the dispersion of the magnesium in the salt melt is operated at a tip speed of about 1,500 to about 4,000 feet per minute, i.e. about 457 to about 1,220 meters per minute. These high stirring speeds are necessary as a result of the high viscosity of the mixtures formed in these processes. These high stirring speeds, of course, mean a relatively high energy consumption to achieve dispersion of the metal in the salt melt.
In order to improve the economics of the process, additional metal is added to the salt mixture, since the initial amount of magnesium in the sludge matrix is normally less than 15% by weight. The maximum amount of magnesium dispersed in the mixture is limited to a maximum of about 42% by weight, and is preferably held to a maximum of about 38-40% by weight. Amounts of magnesium above these limits result in formation of clusters of metal beads adhered to, or coalesced with, each other when cooled, so-called "off-spec" metal.
Furthermore, in both of these processes, the electrolyte salt mixture employed, which contains both alkali metal halogenides and alkaline earth metal halogenides, is hygroscopic, and this makes it necessary to control the relative humidity during handling of the granules to less than 35%, preferably less than 20%.
Thus, it is apparent that there are several disadvantages associated with prior art processes for preparing rotund particles of salt-coated magnesium or magnesium alloy.